This invention generally relates to air/fuel ratio control for an internal combustion engine of vehicles, and more particularly, the present invention relates to such an air/fuel ratio control with a fail-safe device capable of causing the engine to operate during failure of computer control.
Microcomputers are widely used for air/fuel ratio control of an internal combustion engine of motor vehicles or the like. Although the ratio of the air/fuel mixture supplied to an internal combustion engine is optimally controlled on the basis of necessary control information by a microcomputer in conventional computer-controlled engines, once the microcomputer malfunctions fuel supply is interrupted or becomes uncontrollable. In motor vehicles, such undesirable state should be avoided to ensure the safety of passengers. Therefore, some conventional air/fuel ratio control apparatus having a microcomputer is equipped with a fail-safe device as disclosed in Japanese Patent Provisional Publication No. 56-135201 and its corresponding U.S. Pat. No. 4,370,962. According to this prior art a fail-safe device, which operates independently of the microcomputer, is additionally provided so that fuel supply to the engine is continuously ensured even after the microcomputer starts malfunctioning, allowing the engine to continuously operate. As a result, the motor vehicle can be driven, and therefore, it is possible to prevent the motor vehicle from undesirably stopping on a road so that it can be driven it to a nearest service station.
However, in such conventional air/fuel control apparatus, fuel flow is fixed when the fail-safe device operates because fuel injector driving pulses are produced in response to only an engine rotational signal. Namely, the injector driving pulse width is kept constant of the airflow. As a result, although it is possible to drive the motor vehicle at low speeds, such as under 50 Km/h, higher speed driving cannot be expected, while it suffers from unsatisfactory drivability. Furthermore, since the ratio is not positively controlled, the engine is apt to suffer from undesirable combustion, such as misfiring, emission of noxious gasses or the like.
Moreover, since the conventional microcomputer used to determine air/fuel ratio is arranged such that all necessary calculation instructions are programmed in its memory, instructions for deriving a value Q/N by digitally dividing the airflow data Q by the engine speed data N are also prestored in the memory. As a result, a memory having a relatively large storing capacity is required, while a relatively high programming cost is also required.